1. Field of the Invention
This invention relates to solid material mixing and more particularly to such mixing in combination with uniform heat transfer within rotary kilns.
2. Prior Art
Rotary kilns are well known in the art. They generally comprise an elongated, refractory lined cylinder adapted to be rotatably supported with the axis inclined so that various types of granular, particulate materials may, during processing, flow from the upper input end to the lower discharge end of the kiln. Materials commonly processed by pyro processing in rotary kilns include cement, lime, gypsum, etc. The moving materials are commonly heated by counter or concurrently flowing hot combustion gasses, and in the process of progressing through the kiln, the feed material is mixed by the action of the rotation of the kiln. Heat and mixing are critical to achieve the designed uniform, homogeneous end product. As may be appreciated, the form of mixing provided merely by the rotating kiln is highly inefficient, producing a core of non-uniform material. Through the years the state of the art has produced various devices to improve the mixing process and thus produce a more homogeneous product.
Inseparable from the mixing function is the related heat transfer problem. That is to say, a thorough heat transfer from the kiln gasses to the feed mix as well a mixing is also essential to the accomplishment of a high quality homogeneous end product.
There have been many proposals to aid heat transfer from kiln gasses to feed material. These include many well known mechanical devices such as refractory or metallic lifters, or heat exchangers. Another example involves the suspending of chains at a certain point within the kiln for the purpose of increasing mixing and heat transfer. U.S. Pat. No. 3,442,497 to Gantz is an example of a chain mixer. A more recently proposed solution in U.S. Pat. No. 3,030,091 and U.S. Pat. No. 3,169,016 to Wicken et al utilizes multi-chambers within the kiln to improve heat transfer.
Conversely, in the comminution art where the focus is particle crushing, not mixing, ball mills and cinder mills have been known to make use of bars to aid in the comminution process. U.S. Pat. No. 3,318,538 to Needham is an example of a rod utilized in the dry blending of polymers. U.S. Pat. No. 2,868,463 to Hall discloses a ball mill with load dispersion bar which freely rotates within the mill and is utilized to aid in mixing the grinding elements with the feed material. U.S. Pat. No. 841,728 to Sly discloses a horizontal cinder mill with a commutation rod within and extending the length of a rotary drum. In a wet process, U.S. Pat. No. 2,557,528 to Andrews discloses a scraping and tumbling finned member to aid in the continuous digestion of titaniferous material with sulfuric acid. Mixing of the feed is not a focus of these foregoing references and, in fact, segregation of the particles as they are crushed is more often the desired effect.
None of the foregoing address themselves to the exclusive problem of a mixing of a feed where the problem is in trying to disrupt a core of material within the feed itself. Furthermore, a mixing where no comminution is desired is likewise not the prior art focus. Also, none addresses the associated problems where the mixing process is inter-related with a heat transfer process and especially where the latter is a very high temperature operation. Moreover, the comminution art (where most processes are wet processes) does not address itself to the problem introduced where an elongated kiln is used or to a situation where a dry mix is continuously gravity fed and moved through elongated kilns which may exceed 700 feet in length. Certainly the comminution art does not commonly address itself to processes where the feed material is undergoing chemical and/or physical alteration in the course of its progress through the kiln.
Today, rotary kilns, particularly rotary cement kilns, produce a product which is poorly homogeneous because poor mixing and poor heat transfer impede the calcining operation. The end product is often non-homogeneous, poorly formed and has a material segregation which is most notable in that a core remains which is poorly mixed and requires additional heat to achieve proper reaction.